Storage assemblies



April 7, 1970 I M; 1,. wan: 3,505,006

I STORAGE Assmuns Filed Dec. 29, 1966 FIG. 2

ULTRASONIC AGITATION IN TRICHLOROETHYLENE 1 ULTRASONIC AGITATION IN ACEIONE AIR FIRE AT 4ooc.sooc.

4 STACK I PLACE IN 4 CONTAINER 1 LOAD RECOVER lNl/ENTOk M. L. WHITE" IL M ATTORNEY United States Patent Ofi ice 3,505,006 Patented Apr. '7', 1970 Int. Cl. B65d 21/02 Us. (:1. 21-61 13 Claims ABSTRACT OF THE DISCLOSURE Inside a closed and degreased aluminum can, stacked nickel pans having degreased and air-fired surfaces meet each other along their respective side walls and form between their bottoms shallow spaces for holding polished semiconductor wafers.

This invention relates to storage assemblies, particularly for protecting clean metal samples, such as semiconductor wafers awaiting epitaxial deposition, from contamination by hydrophobic organic fluids which are found suspended in industrial atmospheres and which, for example, include the oily aerosol exhausts of vacuum pumps or other machines.

Metal samples, such as wafers awaiting epitaxial deposition, have been protected from contaminating effects of such air-suspended fluids by placing the samples in glass storage jars and sealing the jars from the outer air with Teflon-lined screw caps. These jars have permitted clean storage for anly about 16 hours. Samples stored longer had to be recleaned to be protected. Specifically, after 16 hours the air in the jar had deposited enough hydrophobic organic fluids on the sample surface so that water dIOP'. lets applied to test the surface balled up and thesecalled contact angle measured by a goniometer between the water and the surface of the sample increased from an acceptable 6 degrees indicating a clean surface to an unacceptable result beyond 15 degrees. Such balling-up to a contact angle greater than 15 degrees indicates that the surface is too contaminated for epitaxial deposition.

An improvement in storage time has resulted from packing the clean stamples between two tightly-closed, hermetically-sealed glass plates in suitable shallow depressions within one plate. When such plates are stacked they are known as glass packs. However, even such expedients failed to extend the permissible storage time much beyond one day. Moreover, in all these devices the containers required cleaning just before use.

H An object of the invention is to improve such storage devices particularly by eliminating these deficiencies.

Another object is to protect metal samples from such contamination over much longer periods of time.

Still another object is to simplify the storage means for such samples, particularly by eliminating the need for gastight seals.

Yet another object is to eliminate the need to clean the storage devices just before their use.

According to a feature of the invention these objects are achieved in whole or in part by storing the metal sample between a container and a cover which meet along respective opposing endless bands and whose respective inner surfaces and whose bands are coated with a thermally oxidized metal that is substantially free of hydrophobic organic fluids. The cleanoxidized metal behaves as a getter which contaminating hydrophobic fluids preferentially coat. This reduces the densit of contaminants between the cover and container. New contaminants attempting to enter the container between the opposing bands preferentially coat the band rather than entering the container.

According to another feature of the invention the containers and covers are each identical dish-shaped pans that nest in each others angularly upstanding walls while leaving a small flat volume between them. They allow the cover to be used as a container under a succeeding cover so that a stack of ten pans may hold nine separate samples between them. Preferably the entire assembly is itself stored in a cylindrical oxide-coated, degreased, can meeting an oxide-coated, degreased cover along respective oxide-coated, degreased bands. This furnishes additional gettering for the ambient air surrounding the pans. Such an assembly may be stored or may store samples for a month or more without degrading changes in the contact angle of the pans or the samples. 7

According to yet another feature of the invention, granular oxide-coated material is placed in the container, below a shelf on which the sample rests, to afford additional gettering action.

The invention is based on the observation that organic contamination by air-suspended hydrophobic fluids depends not only on the fluids settling onto a surface due to their random motion, but upon their being preferentially adsorbed by the surface. Specifically, it is based on the observation that oxide films on metal surfaces formed by thermal oxidation become hydrophobic when exposed to air much more rapidly than polished glass. Thus, they tend to purify the atmosphere and significantly reduce the density of such fluids available for contaminating the surfaces of samples to be stored. Moreover, the adsorptive surfaces, when located in air passages to a storage space, substantially eliminate hydrophobic contaminants attempting to enter the storage space.

These and other features of the invention are pointed out in the claims. Other objects and advantages of the invention are available from the following detailed description when read in light of the accompanying drawing wherein:

FIG. 1 is a partially cross-sectional perspective view of an assembly embodying features of the invention;

FIG. 2 is a flow diagram illustrating steps for preparing the apparatus of FIG. 1 for oeration; and

FIG. 3 is a perspective view of another assembly also embodying features of the invention.

In FIG. 1 three stacked storage containers 10 forming a storage assembly 12 accordingto the invention are each composed of a round aluminum base 14 and a pill-box-shaped cover 16. Each base forms a ringshaped pedestal 18 that fits into an annular recess 20 in the cover 16 below it so that the containers can stack as shown. In each base the pedestal 18 integrally suspends a pan-shaped storage section 22 whose bottom surface 23 lies slightly above the bottom of the pedestal 18.

The pan-shaped storage section 22 carries a number of round, nesting, nickel storage pans 24. Respective invefirted' frusto-conical side Walls 26 extend angularly upward and outward from respective flats 28 of the pans 24. The upward angles are such that the side wall 26 of any pan rests on a wide area of the inner surface of the side wall 26 of the pan below without the flats 28 of successive pans touching each other. In particular, the mutually supporting side walls 26 maintain spaces between the flats 18 just large enough for the flats to accommodate the chips 30 of silicon semiconductor mateiral to be stored. They nest in each other so that distances between flats are small.

The lower edges 32 of the covers 16 are tapered-in slightly to fit intimately in the round riser 34 from the pedestal-14 but loosely enough to permit easy removal of the cover 16.

According to the invention, the pans 24, the bases 14 and the covers 16 have inner and outer surfaces coated with .an oxide of the metal from which they are made and are cleaned free of hydrophobic organic contaminants. The oxide is formed and the cleaning of these parts is accomplished according to the flow diagram illustrated in FIG. 2. Here, in steps designated 40 and 42, the storage pans 24 and the containers are first cleaned by ultrasonically agitating them in trichloroethylene and then ultrasonically agitating them in acetone. The thus-degreased parts are then furnished with a heavy oxide coating in step 44 by passing them through a melt furnace at 400 to 500 C. for twenty to thirty minutes-Higher temperatures may be used if they do not melt the material. The preparation continues immediately after this air-firing by stacking the pans- 24, as in step 46, placing them as in step 48 in the containers 10 until they are to be used. The semiconductor slices are loaded as in step 50 by removing the cover 16 from two containers 10 and transferring the pans 24 from one stack in one container to another stack, as the slices of semiconductor material are place in' them. Specifically, this is done by keeping the bottom pan in the storage section 22 of one container 10 and removing the top pan and setting it on a clean surface. The slices arethen loaded into the pans and the top pan placed over the last slice. Using this technique, the slices contact only surfaces that have been protected against contamination. The re-covering technique is designated as step 52.

By virtue of this assembly thus prepared, slices of semiconductor material 30 placed in the pans are exposed only to the atmosphere laden with hydrophobic organic contaminants between the flats 28. The oxides on the upper and lower surfaces of the flats 28 enclosing the small chamber formed for each slice preferentially adsorbs these hydrophobic organic contaminants. The oxides thus behave as a getter. Moreover, contaminants outside the chambers formed by the pans 24 that tend to migrate between the side walls 26 of the pans 24 also find themselves preferentially adsorbed because of the small space between the pan side walls 26 and the comparatively long angular path formed by the nesting side walls.

The container 10 further filters airborne contaminants in the ambient space by adsorbing them on the surfaces in the interior of the base 14 and the cover 16. Other airborne particles outside the containers 10 are adsorbed at the interface between the edge 32 and the rim 34.

According to one example of the invention, the pans 24 were prepared by deep drawing them from sheets of .015 inch of nickel with the sides sloped at an angle of 115 degrees from the flats 28 which were 1.5 inch in diameter. The pans terminated at their edges in outer lips 54 of 2-inch diameter. The pans were onehalf inch high. This sloping furnished the .02-inch spacing between the flats 28 of the pans stacked above each other to afford suflicient room for a .01-inch thick slice of material to be stored. The slices 30 were placed in the bottom of each pan polished side up.

The containers 10, had covers 2.185 inch in diameter and 1.03-inch high made from .OZ-inch thick aluminum. The annular steps were .1S7-inch deep and .109-inch high. The taper at the edges 32 were one and one-half degrees over a .21-inch distance. This cover was accommodated by a base whose risers 34 are 2.1888 inch on the inside diameter and 2.34 inch at the outside diameter. The pan-shaped storage portion 22 corresponded dimensionally to the pans and suspended integrally from a pedestal .38-inch high. The risers may, for example, be .13-inch high.

This structure was then subjected to the process of FIG. 2, namely, ultrasonic agitation in trichloroethllene, and then acetone, air-firing, stacking the pans, placing them in the container, loading the pans with cleaned Nichrome slices and then re-covering the pans. The contact angle changed from an initial angle of 5 degrees to a final angle of 6 degrees in two months.

According to another example, the previously-described assembly of FIG. 1, treated as described in FIG. 2 was loaded with oxidized silicon chips. The initial contact angle of the silicon chip was 7 degrees. After two months the contact angle stayed at 7 degrees.

While in the examples given the pans were made of nickel and the containers of aluminum, any readilyavailable metal that forms an oxide with simple firing and whose charatceristics are not dangerous or incompatible with the material being stored, is suitable. Moreover, the structural parts may include materials other than the metal from which the oxide is grown. Ceramics may be used by coating their surfaces with a suitable metal and oxidizing it. Thermally oxidized metals such as magnesium, aluminum, the transition metals, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, zirconium, niobium, molybdenum, technetium, ruthenium, and other metals such as silver, cadmium, copper and zinc as well as hafnium, tantalum, and tungsten may be used, either alone or on the surface of a structural carrier.

The effectiveness'of the structure in FIG. 1 for contamination-free storage is aided by the close-fitting sides which atford a half-inch length meeting area between adjacent pans.

The container 10 and pans 34 may be used several times for storage when care is used in opening them to the atmosphere for only short periods of time. The gettering action of the pans may be checked at any time by measuring the contact angle of water on the inside surface of the pan. For most effective action this contact angle should be small enough to permit further adsorption. Contact angles less than those exhibited by the pans after two hours in a contaminating atmosphere are desirable. For nickel pans the contact angle should be 12 degrees or less as measured by a goniometer. Recleaning by degreasing and air-firing as set forth in steps 40, 42, 44 and 46 is required when the angle measured is greater than 12 degrees.

A similar action and result may be obtained by storing slices in an aluminum container illustrated in FIG. 3 and having a cylinder and a cover 62. The shelf 64 having openings 66 in the cylinder 60 supports the samples to be protected. The granular metal 68 in the cylinder below the shelf 64 furnishes an additional gettering action. The granular material 68, as well as the entire container shown in FIG. 3, is prepared as shown in FIG. 2. Preferably the container is made from aluminum and the granular material comprises an aluminum material availab e from Reynolds Metals Company, Inc. under designation HPS-lO. This material is a collection of small aluminum flakes having thicknesses of aluminum foil and coated aluminum oxide.

Any metal compatible with the slices 30, especially those previously mentioned, may be used to afford this preferential gettering action.

That the beforementioned oxidized metal surfaces are generally effective for selectively adsorbing air-suspended hydrophobic fluids and hence for gettering these compounds in spaces containing metal samples, is evident from the following experiments.

Ten to twenty-mil thick half-inch squares of aluminum, magnesium, molybdenum, tantalum, nickel, Nichrome, n-type silicon, p-type germanium, zinc, and iron were mounted on a stainless steel block with glyco phthalate and level-lapped with fine grit paper and a garnet of abrasive. The surface was then polished with a slurry of an alumina powder available from the Union Carbide Corp. under the designation Linde A (.3 micron particle size) on billiard cloth until a bright mirror finish was obtained. The slices were demounted by soaking in acetone and then ultrasonically agitated, first in trichloroethylene and then in two successive portions of acetone. The

surfaces were oxidized by heating the samples in a covered nickel container in an air-muffle furnace at 400: 15 C. for 20 to 30'minutes. The contacting angles were checked immediately after the samples were removed from the furnace;

The samples were then subjected to a standard contaminating atmosphere, namely, an atmosphere heavily laden with hydrophobic compounds by placing them on platform above approximately one inch of a minerial oil known as Nujol, held by an aluminum container whose tight fitting top enclosed the samples. Nujol is the trademark of Plough, Incorporated. After two hours, the contact angles for the different metal oxides change as follows:

On the other hand, glass surfaces from microscope slides, precleaned by treatment with chromate-sulphuric acid cleaning solution and through rinsing in overflowing deionized water, and heating the air-muffle furnace, had an initial contact angle of 5 which increased to only 6 in the standard contaminating atmosphere after two hours. Freshly cleaved mica surfaces had contact angles which rose from 3 /2 percent to 7 percent when subjected to the Nujol atmosphere. The absolute rise in contact angle for the oxidized metals was much greater than for glass. This shows that equal surface areas of the oxidized metals were adsorbing larger quantities of organic fluids than the glass.

Similarly cleaned nickel, aluminum and Nichrome oxidized surfaces and freshly cleaved mica surfaces Were tested in laboratory air at intervals over long time periods. The following approximate contact angles were measured:

Contact angle after number of hours (degrees) Metal oxide 0 2 4 6 16 40 64 The figures and curves conventionally derived therefrom indicate that at first the polished metals adsorb hydrophobic organic air-suspended contaminants rapidly in large quantities and then they continue to do so at a less rapid rate but still in substantial quantities. Thus heavy rapid adsorption clears the atmosphere between the nickel pans of air-suspended hydrophobic contaminants and the continuing heavy adsorption after ten hours adsorbs new contaminants attempting to enter the storage space.

This heavy adsorption over short time periods, which nevertheless continues at a less rapid rate, of cleaned thermally-grown oxides on their metals affords the interior of the chambers in FIGS. 1 and 3 their desired gettering action.

While embodiments of the invention have been described in detail, it will be obvious to those skilled in the art that the invention may be practiced otherwise without departing from its spirit and scope.

What is claimed is.

1. An assembly for protecting material from contami nation by hydrophobic substances, comprising container means, closure means covering said container means and forming therewith an interior volume in which the materials to be protected are to be held, said closure means when covering said container means defining in said clo sure means and said container means respective peripherally closed meeting areas opposing each other in close proximity, said meeting areas allowing passage of air between them, said container means and said closure means including a thermally oxidizable metal and defining an interior volume, adsorptive means for preferentially adsorbing hydrophobic fluids within said interior volume, said adsorptive means including a thermally generated oxide coating on the surface of the metal of meeting areas being cleaned to free them of hydrophobic materials.

2. An assembly as in claim 1 wherein said container means has a shelf for holding the materials to be protected.

3. An assembly as in claim 2 wherein said container means contains granular materials having surfaces of metal oxides and being substantially free of hydrophobic materials.

4. An assembly as in claim 1 wherein said container means are dish-shaped and said meeting area on said dish shaped container means are about the interior side walls of said container means.

5'. An assembly as in claim .1 wherein the meeting areas have a dimension along the peripherally endless direction and a second dimension and wherein the sec ond dimension is greater than the interior height of said container means.

6. An assembly for protecting materials from contamination by hydrophobic substances, comprising container means, closure means covering said container means and forming therewith an interior volume in which the materials to be protected are to be held, said closure means when covering said container means defining in said 010 Sure means and said container means respective periph erally closed meeting areas opposing each other in close proximity, said container means and said closure means having surfaces about the entire volume and along the respective meeting areas composed of thermally oxidized metal, said surfaces about the volume and along the respective meeting areas being cleaned and free of hydro phobic materials, said container means and said closure means comprising dishes having respective side walls and nesting in each other, said side walls being sufiiciently steep so that the meeting area extends about the inside side wall of one dish and the outside side wall of the other dish.

7. An assembly as in claim 6 further comprising protective means surrounding said dishes for holding them and having interior surfaces of metal oxide wherein said interior surfaces are substantially free of hydrophobic materials.

8. An assembly as in claim 7 wherein the distance between the bases of the dishes is smaller than the shortest dimension of the contact areas.

9. An assembly as in claim 7 further comprising protecting means identical to said protecting means, said protecting means being mutually stackable.

10. An assembly as in claim 6 further comprising additional nesting dishes having their entire surfaces composed of an oxide from the group of materials consisting of aluminum, magnesium, copper, zinc, titanium, vanadi- 7 um, chromium, manganese, iron, cobalt, nickel, zirconium, niobium, molybdenum, technetium, ruthenium, silver, cadmium, indium, tin, hafnium, tantalum and tungsten, and wherein said oxide surfaces are cleaned to be free of hydrophobic materials.

11. The method of preparing a closea'ble container for sorting materials which comprises, forming in said container along the closure thereof peripherally closed meeting areas which when the container closes are in opposing proximity, producing on the container areas and on the meeting areas surfaces which are adsorptive of hydrophobic compounds; said producing step comprising the subsidiary steps of ultrasonically agitating the container in trichloroethylene, ultrasonically agitating the container in acetone, and forming on the container surfaces and along the meeting areas a metal oxide; and closing the container.

12. The method as in claim 11, wherein the step of forming the container comprises the subsidiary step of forming the meeting areas with a shortest dimension substantially greater than the shortest dimension of the internal volume of the container.

13. The method as in claim 11, which further comprises the step of forming the container with its closure from two dish-shaped members, the step of stacking the members to form the internal volume of the container, and the step of placing the materials to be protected into the container before stacking the members.

8 References Cited UNITED STATES PATENTS 1,141,769 6/1915 Carnahan et al 148-635 1,335,024 3/ 1920 Peschko. 1,532,831 4/1925 Mastin. 2,156,357 5/1939 Simpson. 2,842,223 7/1958 Zall 55-387 2,882,244 4/ 1959 Milton. 2,967,119 1/1961 Gutterman 134-1 2,970,886 2/1961 Keeve 55-74 XR 2,979,157 4/1961 Clark 55-387 3,124,671 3/ 1964 Juptner. 3,214,381 10/1965 Baldauf 2060.4 XR 3,182,668 5/1965 Hartsell 134-1 XR 3,227,273 1/ 1966 Syverson et al. 3,265,239 8/1966 Kohan et a1. 220-64 OTHER REFERENCES National Bureau of Standards Circular 566', Bibliography of Solid Adsorbents 1943 to 1953, articles 7510, 1953, 1994.

MORRIS O. WOLK, Primary Examiner D. G. MILLMAN, Assistant Examiner U.S. Cl. X.R. 

